AbstractThe seasonal cycles of the zooplankton in 1967–1969 in Lakes Rotorua and Rotoiti in the North Island of New Zealand are outlined.Bosmina meridionalishad several peaks in abundance during the summer and low winter numbers, whilstCeriodaphnia dubiagenerally had only a single period of real abundance during the summer. The rotiferAsplanchna priodontawas present throughout the year in low numbers except for two brief but striking peaks in the spring‐summer period. The only cyclopoid copepod present wasMacrocyclops albiduswhich was never abundant.The annual cycles of the calanoid copepodCalamoecia lucasiwere studied in some detail. Total numbers were high but very variable during spring‐autumn and low in winter, the seasonal cycles being very similar in both lakes. Breeding was continuous although egg clutches were small (1.1–6.6), and the development of at least 5 cohorts per year could be followed from data on breeding intensity, population age‐structure and adult size. Laboratory determined egg development rates were used to calculate birth rates in the populations so that production and losses could be estimated. Survival rates appeared to be higher in Rotoiti than in Rotorua. Birth and death rates were low, the annual means for both linear and exponential rates ranging from around 0.04 to 0.07, giving turnover times for individuals of 12–18 days in Rotorua and 18–24 days in Rotoiti. Population turnover rates were higher in Rotorua (19–28 days), which had a similar mean population per square metre but higher clutch sizes, than in Ro